In liquid crystal display devices such as liquid crystal television, for example, liquid crystal panels that are display panels do not emit light. Therefore, liquid crystal display devices require backlight devices as external lighting devices. The backlight devices are classified broadly into a direct type and an edge-light type based on mechanisms. For further reduction in thicknesses of the liquid crystal display devices, the edge-light type backlight devices are more preferable.
In an edge-light type backlight device, alight guide plate has a recess (or a projection), which is a cutout, on its edge surface and a casing has a projection (or a recess) in a part thereof and corresponding to the recess (the projection) of the light guide plate. It has been known that the light guide plate is positioned within the casing with respect to a plate surface of the casing by fitting the recess and the projection each other. Patent Document 1, for example, discloses a lighting device including such a configuration in which the light guide plate is positioned.